How do I assign a second value in cel of a 2D array?

I am trying to find a method to include a second variable in a 2D array.
Additionally I only want to assign a second variable to cells that allready have one.
for instance, I start with the array:

seq = [[1, 0, 0, 0],[1, 2, 3, 4],[2, 0, 0, 0]]

and I want to assign a second integer to the values that allready have one, making this:

seq = [[(1, a), 0, 0, 0],[(1, b), (2, c), (3, d), (4, e)], [(2, f), 0, 0, 0]]

in which I ideally want a loop in which i can select the particular values with a rule. I don’t know for sure I can keep the 0’s in the particular positions without assigning a second value to them, or if the array needs to be converted to a list.
I’m quite new to python and normally work with Matlab.

>Solution :

Try:

values = list('abcdef')
result = list()
for row in seq:
    result.append([(n, values.pop(0)) if n!=0 else 0 for n in row])

>>> result
[[(1, 'a'), 0, 0, 0],
 [(1, 'b'), (2, 'c'), (3, 'd'), (4, 'e')],
 [(2, 'f'), 0, 0, 0]]